Assisted steering system for non-trackbound vehicle

ABSTRACT

Steering is actively assisted in a turning motor vehicle including a hybrid drive having both an internal combustion engine and a generator unit. The inner and outer wheels are each associated with an individual drive which is controlled in accordance with the steering angle of the steered wheels. Specifically, the torque differential is adjusted such that the percentage actual wheel differentials approach the percentage reference wheel speed differentials. A substantial reduction in the force which must be applied via the steering system to adjust the steering wheels, as compared to an absence of torque differential, is achieved because the outer wheel is driven at a higher output than the inner wheel when making a turn.

1. Field of the Invention

The invention relates to non-trackbound or non-railborne vehicles withat least two pairs of wheels, generally, and more particularly tovehicles in which the left and right wheel of at least one of the pairscan be adjusted by a steering wheel and in which each wheel of at leastone of the pairs is individually driven by a corresponding drive.

2. Description of the Prior Art

Assisted steering (servo-steering) is commonly used in non-railbornevehicles, i.e. passenger cars, trucks, busses and the like, which aregenerally driven by means of internal combustion engines. A hydraulicpump driven by the internal combustion engine serves as an energy sourceproviding the power required for boosting the force applied to thesteered wheels by the operator of the vehicle via the steering wheel. Inaddition to the hydraulic pump, control valves, steering valves,steering cylinders, an oil tank and line connections are required.Accordingly, assisted steering systems were formerly very costly.

More recently, new drive concepts for vehicles have been developed whichdiverge sharply from the internal combustion engine drives commonlyknown for decades. For example, non-railborne vehicles withbattery-powered electric motor drives have been suggested. There havealso been suggestions for hybrid drives in which an internal combustionengine/generator unit is provided at the vehicle and the electricalpower supplied by the generator is fed via power electronics to electricmotors which are rigidly coupled with the driven wheels. Such driveshave certain advantages, since they allow differentiated steering aspure internal combustion engine drives.

A substantial feature of vehicles with electric motors driving at leasttwo wheels consists in that the driven wheels each have their ownelectric motors.

A key element in the gradual conversion of motor vehicles driven only byinternal combustion engines to electric vehicles or vehicles with ahybrid drive consists in adapting the handling of these new vehicles tothe handling of vehicles driven by internal combustion engines. In termsof the vehicle steering system, this would mean providing assistedsteering, since substantial resistance must be overcome particularly inheavy vehicles with rubber tires. Conventional techniques for assistedsteering are provided for this purpose, i.e., in the case ofelectrically driven hydraulic motors, conventional servo-steering couldbe applied. The steering could also be effected by means of electricmotors, i.e. an electric motor is actuated depending on the steeringwheel position in order to move the steering linkage. However, this typeof assisted steering is also quite expensive.

A vehicle drive is known from JP 62-181 918 A in which a mechanicaltransmission controlled via hydraulic clutches is provided for the rearaxle of the vehicle, by means of which the distribution of torquebetween the two driving wheels on the left and right sides of the rearaxle can be influenced depending on the speed of the vehicle and theangle and actuating speed of the steering wheel in such a way that thedriving wheel on the outside of the turn receives greater torque thanthe inner driving wheel. The required cost for this construction isextremely high. More detailed information regarding the appliedregulating strategy in the torque adjustment is not provided in thisprior art.

When making a turn, the speed of the outer wheel is naturally greaterthan that of the inner wheel. The so called single-track model serves asa basis for calculation, whereby with reference to the accompanying FIG.1 with a reference turning radius R, the "Ackermann" steering angleδ_(A) depending on the steering wheel angle is calculated as follows:##EQU1## with

    1.sub.2 =(m.sub.v /m.sub.g)·1                     (2),

where

1=axial distance

2=distance of the center of gravity SP from the rear axle

R=reference turning radius

δ_(A) =steering angle

m_(V) =weighted front wheel axle load

m_(g) =total weight of vehicle.

It will be seen that the percentage wheel speed differential between theouter and inner wheels of the vehicle can be determined by thissingle-track model allowing for the track width of the vehicle.

Theoretically, a linear relationship is given when the percentage wheelspeed differential between the outer and inner wheel is applied to thewheel in question via the steering wheel; the percentage wheel speeddifferential is approximately 10% at a steering angle of 10° at thewheel.

In a vehicle having a single-wheel drive, the drive output is determinedby the driver by actuating the accelerator pedal. This drive output mustbe transmitted in its entirety to the driving wheels. Thus, the totaldrive output determined by the driver is the sum of the single-wheeldrive outputs.

SUMMARY OF THE INVENTION

The aforementioned object, as well as others which will become apparentfrom the description provided herein, is met by a vehicle in which thetorque transmitted to the outer wheel or the output transmitted thereto(the output is in a linear relationship with the torque by way of thespeed) is increased depending upon the steering wheel angle andtherefore upon the steering angle at the wheel, whereas the torque oroutput at the inner wheel is decreased to a corresponding degree.

The driver of the vehicle positively perceives the change in drivingtorque of the outer and inner wheel in an easy steerability of thevehicle.

A particular advantage of the invention consists in that the individualdrives for the driven wheels are constructed as electric motors and arealready separately controlled. The steering angle can therefore be takeninto account in the control of the power supply to the driving electricmotors in a very simple manner.

According to the basic ideas of the invention, the torques applied tothe driven wheels are increased and reduced depending on thetheoretically determined percentage wheel speed differential dependingon the steering angle. This applies particularly for relatively lowvehicle speeds (e.g. less than 50 km/h) at which the theoretical valuesare retained entirely or almost entirely (e.g. 80 or 90%).

However, based on general experience and viewpoints according to theinvention, an increasing deviation from the linear characteristic linebased on the "Ackermann" theory is provided at higher vehicle speeds.

FIG. 3 shows the linear theoretical "Ackermann" relationship between thesteering wheel angle δ and the percentage wheel speed differentialbetween the right and left wheels in the form of a straight line A. Whenmaking a turn, the driver turns the steering wheel of the vehicle. In sodoing, certain speeds and consequently certain speed differentials areadjusted in the steered wheels. The speeds of the wheels can be detectedby sensors and a deviation between the actual differential and thereference differential can be calculated according to FIG. 3 based onthe determined differential. This deviation is used as a controlvariable for the increase or decrease in driving torque for theindividual outer and inner driving wheels.

It is known that at extremely slow vehicle speeds, the actual wheelspeed differential deviates slightly from theory. At higher vehiclespeeds, the actual wheel speed differentials always diverge more sharplyfrom the theoretical values. This is shown in FIG. 3 by a family ofcharacteristic lines with a vehicle speed v as parameter.

Curve B in FIG. 3 shows the qualitative actual wheel speed differentialas a function of the steering wheel angle at a vehicle speed of 50 km/h,the characteristic lines C and D are representative qualitatively forvehicle speeds of 100 km/h and 150 km/h.

If the adjustment of the various driving torques of the outer and innerwheels were oriented exclusively to the theoretical characteristic lineA, the difference in the driving torques would be too great at highvehicle speeds, resulting in an unusually high degree of assistedsteering which the driver would perceive as oversteering.

Consequently, the invention provides that the individual drives areadjusted in such a way that the percentage wheel speed differentialsbetween the outer and inner wheels approximately satisfy thetheoretical, geometrical "Ackermann" relationship, at least at lowvehicle speeds, wherein the difference between the actual wheel speeddifferential and the theoretical reference wheel speed differential isused as a control variable, while there is an increasingly sharpdeviation from the geometrical relationship at higher vehicle speeds.

In an advantageous further development of the invention, the differencesin torque between the outer wheel and the inner wheel should be higher

a) at higher steering wheel angle speeds and/or

b) at greater steering wheel angles.

The adjustment of smaller wheel speed differentials at higher vehiclespeeds by relatively small differences in torque is shown in the familyof characteristics shown in FIG. 3. This gives the curve of theproportions of the theoretical reference wheel speed differentialsadjusted by controlling the vehicle drive shown in FIG. 4. In a vehiclespeed range approaching zero, the adjustment of the wheel speeddifferentials is 100% corresponding to the theoretical characteristicline A or, e.g., characteristic line A' in FIG. 3 which deviates onlyslightly from the latter. As the speed of the vehicle increases, thedeviation from the theoretical characteristic line is allowed toincrease continuously. Thus, the respective speed differential becomesincreasingly smaller, i.e. only a small proportion of the theoreticalreference speed differential is adjusted. At high vehicle speeds, thereis practically no assisted steering; the steering is virtually direct orimmediate.

In this context, low speed refers to a speed below 40 km/h. Until thispoint, the aimed for speed differential (reference wheel speeddifferential) may be no more than a maximum of 20% less than thetheoretical "Ackermann" speed differential. This deviation should beless than 10% below a speed of e.g. 25 km/h. At speeds below 10 km/h,the speed differential according to "Ackermann" should be retained asfar as possible. On the other hand, at higher vehicle speeds, only afraction of the theoretical "Ackermann" differential is adjusted, forexample less than 60% of this differential at a speed of more than 100km/h.

In accordance with an illustrative embodiment of the present inventionthe difference in the driving torques applied to the individually drivenwheels increases at higher steering wheel angle speeds. That is, whenthe steering wheel is turned very quickly, a very high degree ofassisted steering is effected. This relationship is shown in FIG. 6, inwhich the torque differential is shown over the steering wheel anglespeed δ as a qualitative function. Other steadily increasing functionalcurves (e.g. degressive or progressive) can also be used.

The torque differentials are also dependent on the steering wheel angle.With a greater turning of the steering wheel or higher steering wheelangles, the difference in torque between the outer wheel and inner wheelshould also be greater according to alternative b). According to FIG. 7,this dependence is not linear, but is somewhat flattened at greatersteering wheel angles. Accordingly, when parking the vehicle, forexample, a very high degree of assisted steering is advantageouslyeffected when the steering wheel is turned by a large amount so that,e.g., parallel parking is very convenient. When the steering wheel ismoved very quickly in addition, a high responsiveness of the assistedsteering is achieved by means of the temporary overproportionateassisted steering as a result of a temporarily great torque differentialbetween the outer and inner driving wheels.

FIG. 7 also shows the dependence of the change in torque on the vehiclespeed. Curve X represents low vehicle speeds, curve Y represents mediumspeeds, and curve Z represents high vehicle speeds.

When the above aspects are considered in combination, a torquedifferential δM between the outer wheel and inner wheel for a change inthe steering wheel angle δ over time results as shown by way of examplein FIG. 5. At first, the steering wheel angle 6 changes slowly butsteadily, assuming a constant mean vehicle speed. Accordingly, thetorque differential, i.e. the extent of assisted steering, also changes.In a segment Δt, the steering wheel rotation is accelerated.Consequently, there is also an increase in the torque differential ΔMand accordingly in the assisted steering according to FIG. 6, since theangle speed of the steering wheel is increased. Further, the harderturning of the steering wheel according to FIG. 7 results in anadditional increase in the assisted steering. In FIG. 5, the torquedifferential ΔM is characterized by a sharply ascending segment. Whenthe movement of the steering wheel slows down again, as shown in FIG. 5by a gradually flatter slope of the curve δ, the assisted steering dropssomewhat. According to the characteristic field shown in FIG. 7, thereis hardly any further increase in the assisted steering as the steeringwheel angle δ continues to increase in a uniformly continuous manner,which is attributable to the flattening of the family of curves in FIG.7 at higher values of δ.

Thus, with a given individual wheel drive, the invention makes itpossible to realize an assisted steering system at an extremely low costand steps may be taken, also at a low cost, which permit a simple andreliable steering of the vehicle in practically any situation.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of the disclosure. For a better understanding of the invention, itsoperating advantages, and specific object attained by its use, referenceshould be had to the drawings and descriptive matter in which there areillustrated and described preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present invention will be explained inmore detail in the following with reference to the drawings, in which:

FIG. 1 is a graph illustrating the calculation of the wheel speeddifferentials in a vehicle while making a turn;

FIG. 2 shows a schematic diagram of a vehicle which is outfitted with aninternal combustion engine/generator unit (VGE) and with assistedsteering according to the invention;

FIG. 3 is a graph showing the dependence of the wheel speed differentialon the steering wheel angle;

FIG. 4 is a graph representing the compensation of the wheel speeddifferentials depending on the vehicle speed;

FIG. 5 is a graph showing the relationship between the steering wheelangle and the torque differential generated for assisted steering at theindividually driven wheels of the vehicle according to FIG. 2 as afunction of time;

FIG. 6 is a graph showing the relationship between the steering wheelangle speed and the torque differential for the individually drivenwheels;

FIG. 7 is a graph showing the dependence of the torque differentialgenerated at the individually driven wheels with the vehicle speed asparameter depending on the steering wheel angle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a schematic view of a passenger automobile 2 with a non-drivenrear wheel pair and a pair of driven front wheels which are adjusted inthis instance to a steering angle δ_(A) for a left-hand turn.

The outer front wheel 4 with respect to the turn is driven by anelectric motor 8 which is coupled with it, the inner front wheel 6 isdriven by an electric motor 10 coupled therewith. A steering linkage ofconventional construction is coupled via a steering gear 12 and asteering column 22 with a steering wheel 20. A turning δ of the steeringwheel results in a steering angle δ_(A) of the front wheels 4 and 6.

The vehicle 2 has an internal combustion engine/generator unit (VGE) 16as power source. The generator part supplies electric current to a powerdistributor 14 outfitted with power electronics. The power distributor14 controls the power supply to the two electric motors 8 and 10individually via feed lines L1 and L2. The power distributor 14, VGE 16and other components of the vehicle 2 not of concern in the presentcontext are controlled by a control unit 18 which is equipped with amicroprocessor and receives the sensor signals of different sensors forthis purpose.

In particular, the control unit 18 receives wheel speed signals whichare tapped at the electric motors 8 and 10 coupled with the front wheels4 and 6. The signal paths are shown in FIG. 2.

Moreover, the control unit 18 receives a position signal representingthe steering wheel angle or steering wheel turn δ by means of a sensor24 arranged in the region of the steering system.

From a throttle, not shown, the control unit 18 receives a signalcharacterizing the output demanded by the driver. Accordingly, thecontrol unit 18 controls the VGE 16 and the power distributor 14 inorder to make the output required by the driver available to the wheels4 and 6 via the electric motors 8 and 10. Depending on the steeringangle δ_(A) and consequently on the steering wheel angle δ, the controlunit 18 generates a driving torque differential corresponding to apercentage wheel revolution differential between the outer wheel 4 andinner wheel 6 via the power distributor 14. The output and torque at theelectric motor 8 coupled with the outer wheel 4 is increased by acertain amount based on the overall drive output, and the output of theelectric motor 10 coupled with the inner wheel 6 is reduced (e.g. by anequal amount).

In a modified embodiment, each of the four wheels of the vehicle can beoutfitted with its own electric motor. All wheels can be then controlledindividually when turning a corner.

In another modified embodiment, the rear wheels of the vehicle can bedriven, while the front wheels merely run along and are coupled with thesteering. The individually driven wheels can be identical to the steeredwheels, but this is not necessary.

The sensor 24 shown in a simplified manner at the steering wheel in FIG.2 is advisably arranged at the steering gear 12 or at the steeringlinkage.

As already mentioned, the percentage wheel speed differential whencornering corresponds in theory to a straight line A in FIG. 3. At verylow speeds, a torque distribution is carried out--depending on thesteering wheel angle δ--in such a way that the reference wheel speeddifferential according to the straight line A in FIG. 3 is achieved withthe driving torques applied to the individually driven wheels. In otherwords, the control variable is the difference between the referencewheel speed differential according to the straight line A in FIG. 3 andthe actual wheel speed differential detected by the control unit 18 viathe sensors at the electric motors 8 and 10.

As a result of the adjustment of the driving torque differential whencornering, the outer side of the vehicle is driven at greater power thanthe inner side. In extreme cases, this torque distribution leads to abraking of the inner wheels or inner wheel. The steering is activelyassisted by means of the torque distribution: the required force to beapplied for moving the steering linkage is considerably smaller thanwould be the case if no torque differential were adjusted whencornering.

As already mentioned above, the adjustment of the torque differential iseffected as a function of the vehicle speed. The higher the vehiclespeed, the smaller the differential. This is shown schematically inFIGS. 3 and 4.

Further, the extent of assisted steering, i.e. the extent of the torquedifferential, is effected as a function Of the steering wheel positionand the steering wheel angle speed as is shown in FIGS. 6 and 7.

Of course, the present invention is also applicable in vehicles withmore than one pair of steered wheels, e.g. in passenger automobiles withsteered front and rear axles.

We claim:
 1. A vehicle comprising:at least two pairs of wheels having aleft and right wheel, at least one of said pairs of wheels being coupledto a steering wheel manipulable into a steering angle for turning thevehicle: a plurality of electric drive motors, each respective drivemotor being operable to independently drive a corresponding one of saidwheels such that during a turn one of said drive motors is associatedwith an inner wheel and another of said drive motors is associated withan outer wheel; and steering assisted means for controlling said drivemotors in accordance with steering wheel angle, said steering assistingmeans operating the drive motor associated with the outer wheel at ahigher torque than the drive motor associated with the inner wheel toachieve a percentage wheel speed differential between said inner andouter wheels, wherein said drive motors are adjusted at lower vehiclespeeds to achieve percentage wheel speed differentials in accordancewith a predetermined linear relationship between steering wheel angleand percentage wheel speed differential and at higher vehicle speeds toachieve percentage wheel speed differentials which are increasinglysmaller with increasing speed.
 2. The vehicle according to claim 1,wherein said assisting means is operable to adjust the individual drivesin such a way that the torque differentials between the outer and innerwheel increase as the steering wheel angle increases.
 3. The vehicleaccording to claim 1, further including an internal combustionengine/generator unit and means for distributing electrical powersupplied by the generator to the electric motors in a controlled manner.4. The vehicle according to claim 1, further including a steering wheelsensor for detecting steering wheel position.
 5. The vehicle accordingto claim 1, wherein a difference between an actual wheel speeddifferential and a theoretical reference wheel speed differentialderived in accordance with said linear relationship is used as a controlvariable.
 6. The vehicle according to claim 1, wherein said steeringassisting means is operable to adjust the individual drive motors insuch a way that the torque differentials between the outer and innerwheel increase as steering wheel angle speed increases.